Passive infrared intrusion detection systems are known for sensing the presence of an intruder in a protected space and providing an output signal representative of intruder detection. Examples of passive infrared intrusion detection systems are shown in U.S. Pat. Nos. 3,036,219; 3,524,180; 3,631,434; 3,703,718; and 3,886,360. It is an object of the present invention to provide a system and a mirror assembly therefor especially suited to ceiling mounting or high wall mounting to produce a protective curtain through which an intruder must pass to gain access to a protected facility.